A color thermal printer using a color thermosensitive recording paper is known. The color thermosensitive recording paper comprises three thermosensitive coloring layers which are formed on a support to color in yellow, magenta and cyan respectively. The color thermal printer colors the color thermosensitive recording paper by heating it with a thermal head. The color thermal printer records images of the respective colors in a frame sequential manner so that a full-color image of one frame is obtained. In the thermal head, for example, a plurality of heating elements corresponding to pixels are aligned in a scanning direction. Thermal recoding is performed while the color thermosensitive recording paper is moved in a feeding direction in a condition that the thermal head is pressed against the color thermosensitive recording paper.
The color thermosensitive recording paper is moved by a feed roller. For example, the feed roller is driven by a stepping motor, rotation of which is transmitted to the feed roller via a reducer. As to this reducer, a traction-driven type (see Japanese Patent Laid-Open Publication No. 11-146676) is used for instance. As is well known, the reducer of the traction-driven type comprises a sun roller to which the rotation of the motor is inputted, and planetary rollers disposed around the sun roller. The planetary rollers slide on and revolve about the sun roller. The reducer of the traction-driven type further comprises an output shaft driven by the revolution of the planetary rollers.
The reducer of the traction-driven type has lesser backlash in comparison with a reducer of a gear type. In addition, speed fluctuation is also lesser. Consequently, when the reducer of the traction-driven type is employed in a system for driving the feed roller of the printer, speed fluctuation of the recording paper is restrained. Thus, there is an advantage that color unevenness is restrained. Meanwhile, in the above-mentioned Publication No. 11-146676, a roller to which the motor transmits the rotational force via the reducer is provided with a rotary encoder. Encoder pulses generated by the rotary encoder are detected to perform feedback control for a rotating speed of the motor so that rotational unevenness of the roller is restrained.
As for the reducer of the traction-driven type, although the rotational unevenness is restrained, there is a problem in that reduction ratios of the respective reducers are likely to be different due to variation in accuracy of the sun roller and the planetary rollers, wherein the variation in accuracy is caused at a time of production. Moreover, if load fluctuation occurs, there arises another problem in that the reduction ratio is likely to be changed due to slipping caused between the sun roller and the planetary roller. When the reduction ratios of the reducers are different, a feed amount of the recording paper becomes different in the perspective printers. In this case, variation in printing length of a print image is caused. Further, if the reduction ratio is changed due to the load fluctuation, variation of the printing length is caused relative to the print images printed by a single printer.